1. Field of the Invention
The present invention relates to a common mode inductor, and more particularly, to a common mode inductor which is constructed to be used as a noise filter, for example.
2. Description of the Related Art
A conventional common mode inductor of this type is, for instance, a multilayer common mode inductor having an electrical equivalent circuit arrangement shown in FIG. 4. This common mode inductor includes a coil La which is connected between an input electrode 1a and an output electrode 1b, a coil Lb which is connected between an input electrode 2a and an output electrode 2b, and a coil Lc which is connected between an input electrode 3a and an output electrode 3b. The coils La, Lb and Lc are electromagnetically coupled together and are arranged to prevent passage of common mode noise in the same phase.
In this type of common mode inductor, the lower the dc resistance values of the three coils La-Lc, the better the signal propagation characteristics of the inductor. Therefore, in the conventional common mode inductor, a material having low resistivity is used to form the coil conductors which define the coils La-Lc, and the dc resistance values of all the coils La-Lc are lowered by increasing the thickness of the coil conductors.
However, the importance of the dc resistance values of the three coils La-Lc is sometimes different, depending on the circuit in which the common mode inductor is used. For instance, consider a case where the common mode inductor is used as a filter for eliminating noise in an audio circuit of an audio device, such as a stereo. An audio circuit is a three-wire circuit which includes left and right signal lines and a common ground line. Normally, the coils La-Lc of the common mode inductor are arranged in a parallel configuration, and the coils La and Lc on each end are respectively connected to the left and right signal lines of the audio circuit. The coil Lb located in the center of the three coils La-Lc is connected to the common ground line of the audio circuit. The purpose of this type of connection is to ensure that the electromagnetic coupling between the left signal line and the ground line is substantially equal to the electromagnetic coupling between the right signal line and the ground line.
When the in-phase inductor is connected to the audio circuit in this manner, a phenomenon known as crosstalk may occur, in which signals leak between the left and right signal lines. Crosstalk is mainly caused when the center coil Lb, which is connected to the ground line, has a high dc resistance value, which acts as what is referred to as a common-mode impedance of the left and right sides of the audio circuit. On the other hand, the dc resistance values of the coil La and the coil Lc have almost no effect on generating crosstalk. In other words, when a common mode inductor is used as a filter for eliminating noise in an audio circuit, it is only necessary to lower the dc resistance value of the centrally located coil Lb which is connected to the ground line of the audio circuit.
Therefore, when an expensive material having low resistivity is used to reduce the dc resistance values of all the coils La-Lc, the manufacturing cost increases and the more expensive, low resistivity material is also wasted. In addition, increasing the thickness of the coil conductors is likely to cause separation of layers (delamination) when the laminated body is fired.